The present invention relates to an improved method for predicting the microstructure with which a certain cast iron melt will solidify. The invention also relates to an apparatus for carrying out the method.
WO86/01755 (incorporated by reference) discloses a method for producing compacted graphite cast iron by using thermal analysis. A sample is taken from a bath of molten cast iron and this sample is permitted to solidify during 0.5 to 10 minutes. The temperature is recorded simultaneously by two temperature responsive means, one of which is placed in the centre of the sample and the other in the immediate vicinity of the vessel wall. So-called cooling curves representing temperature of the iron sample as a function of time are recorded for each of the two temperature responsive means. According to this document it is then possible to determine the necessary amount of structure-modifying agents that must be added to the melt in order to obtain the desired microstructure. However, no detailed information is given about how to evaluate the curves.
WO92/06809 (incorporated by reference) describes a specific method for evaluating the cooling curves obtained by the method of WO86/01755. According to this document, an early plateau in the cooling curve indicates that flake graphite crystals have precipitated close to the temperature responsive means. As the sample vessel is intentionally coated with a layer of oxide or sulfide-bearing material which consumes the active form of the structure-modifying agents, and thus simulates its natural loss or fading during the casting period, such a plateau can often be found in a cooling curve from a temperature responsive means arranged close to the vessel wall. The skilled person can then determine whether any structure-modifying agent has to be added to the melt in order to obtain compacted graphite cast iron by using calibration data.
The method of WO92/06809 requires xe2x80x9cperfectxe2x80x9d curves comprising a distinct plateau. However, sometimes cooling curves without a distinct plateau are recorded despite the fact that flaky graphite has been formed. Up to now, it has not been possible to use curves without a distinct plateau as a basis for calculating the precise amount of structure-modifying agent that must to be added to the melt in order to produce compacted graphite cast iron over the entire casting period.
Now, it has turned out that it is possible to use virtually any set of cooling curves obtained for eutectic and under-eutectic solidification, and by the equipment of WO86/01755 and WO92/06809 as a basis for calculating the precise amount of structure-modifying agent that must be added. The method of the present invention comprises the steps of:
a) determining the amount of structure-modifying agent that has to be added to the melt in order to obtain compacted graphite cast iron, or spheroidal graphite cast iron, as a function of xcex3, where
xcex3=(TAmaxxe2x88x92TAmin)/(TBmaxxe2x88x92TBmin)
xe2x80x83and wherein
TAmax is the local maximum value of the cooling curve recorded at the centre of the sample vessel;
TAmin is the local minimum value of the cooling curve recorded at the centre of the sample vessel;
TBmax is the local maximum value of the cooling curve recorded at the sample vessel wall;
TBmin is the local minimum value of the cooling curve recorded at the sample vessel wall;
b) determining the amount of structure-modifying agent that has to be added to the melt in order to obtain compacted graphite cast iron, or spheroidal graphite cast iron, as a function of xcfx86, where
xcfx86=(TAxe2x80x2max/(TBxe2x80x2max)
xe2x80x83wherein
TAxe2x80x2max is the maximum value of the first derivative of the cooling curve recorded at the centre of the sample vessel; and
TBxe2x80x2max is the maximum value of the first derivative of the cooling curve recorded at the sample vessel wall;
c) determining the amount of structure-modifying agent that has to be added to the melt in order to obtain compacted graphite cast iron, or spheroidal graphite cast iron as a function of the area (xcfx81B) of the first peak of the first derivative of the cooling curve recorded at the sample vessel wall;
d) determining the amount of structure-modifying agent that has to be added to the melt in order to obtain compacted graphite cast iron, or spheroidal graphite cast iron as a function of xcexa, where:
xcexa="sgr"A/"sgr"B
xe2x80x83wherein
"sgr"A is the area under the second peak of the first derivative of the cooling curve recorded in the centre of the sample vessel; and
"sgr"B is the area under the second peak of the first derivative of the cooling curve recorded at the vessel wall;
e) recording cooling curves at the centre of the sample vessel and at the sample vessel wall, respectively, for a particular sample of a molten cast iron;
f) depending on the result in e) choosing one of the calibration curves from step a)-d) giving the most accurate result; and
g) calculating the amount of structure modifying agent that has to be added to the melt.
As already mentioned, the present invention relates to an improved method for predicting the microstructure in which a certain cast iron melt will solidify. By using the present method, it is possible to evaluate a much larger range of temperature time curves compared to the state of the art and it is also possible to obtain more accurate results.
The term xe2x80x9ccooling curvexe2x80x9d as utilized herein refers to graphs representing the temperature as a function of time, which graphs have been recorded in the manner disclosed in WO86/01755 and WO92/06809.
The term xe2x80x9csample vesselxe2x80x9d as disclosed herein, refers to a small sample container which, when used for thermal analysis, is filled with a sample of molten metal. The temperature of the molten metal is then recorded during solidification in a suitable way. The walls of the sample vessel are coated with a material which reduces the amount of structure-modifying agent in the melt in the immediate vicinity of the vessel wall. Preferably the sample vessel is designed in the manner disclosed in WO86/01755, WO92/06809, WO91/13176 (incorporated by reference) and WO96/23206 (incorporated by reference).
The term xe2x80x9csampling devicexe2x80x9d as disclosed herein, refers to a device comprising a sample vessel equipped with at least one temperature responsive means for thermal analysis, said means being intended to be immersed in the solidifying metal sample during analysis, and a means for filling the sample vessel with molten metal. The sample vessel is preferably equipped with said sensors in the manner disclosed in WO96/23206.
The term xe2x80x9cstructure-modifying agentxe2x80x9d as disclosed herein, relates to compounds either promoting spheroidization or precipitation of the graphite present in the molten cast iron. Suitable compounds can be chosen from the group of inoculating substances well-known in the art, and shape-modifying agents, such as magnesium, cerium and other rare earth metals. The relationship between the concentration of structure-modifying agents in molten cast irons and the graphite morphology of solidified cast irons have already been discussed in the above cited documents WO92/06809 and WO86/01755.
The invention also relates to an apparatus for controlling the production of compacted graphite cast iron, which apparatus takes a sample of molten cast iron, uses the present method for calculating the necessary additions, if any, of structure-modifying agents to the molten cast iron, and provides the molten cast iron with said amount of structure-modifying agents. The apparatus comprises a sampling device, a computer-based data acquisition system, and a means for administrating structure-modifying agents to the molten cast ron. The sampling device contains a representative sample of the molten cast iron which is subjected to thermal analysis during which temperature/time measurements are transmitted to a computer and presented in the form of cooling curves. The computer calculates the necessary amount of structure-modifying agent that must be added and automatically actuates the means for administrating the structure-modifying agent, whereby the melt is supplied with an appropriate amount of such agents.